Carbon monoxide (CO) is used widely in the chemical industry as a raw material in the synthesis of various chemicals including acetic acid, polyurethane foams and polycarbonate plastics. The primary method for production of CO is the steam reforming of methane to produce a synthesis gas mixture. The key components of the synthesis gas include CO, H.sub.2, CO.sub.2, CH.sub.4, N.sub.2 and H.sub.2 O. This synthesis gas mixture must then be separated to produce a high purity CO product. There are three basic methods for the separation and purification of CO including cryogenic distillation, absorption (COSORB) and pressure swing adsorption. Pressure swing adsorption processes for the production of CO utilize adsorbents that have high adsorption selectivity for CO over H.sub.2, CH.sub.4, N.sub.2 and CO.sub.2. However, many CO selective adsorbents consist of CuCl supported on inorganic materials, like alumina, and are adversely effected by water vapor. Firstly, water vapor is strongly adsorbed by inorganic-based adsorbents and thereby reduces the effective CO capacity of the adsorbent. Secondly, water vapor may oxidize the active CuCl to inactive Cu(OH)Cl. Finally, water may also solubilize chloride containing species on the adsorbent surface and cause corrosion problems. Hence, the effective removal of water during the pressure swing adsorption production of CO is a difficult problem that must be resolved for the technology to be successful.
In a paper by Kasuya and Tsuji (Gas Separation and Purification, Volume 5, page 242, December 1991), a PSA process developed by Kobe Steel and Kansai Coke for the production of CO is described. In this process, water vapor is removed in a two-bed thermal swing adsorption system prior to a four bed PSA for the production of CO. Thus, water removal is accomplished prior to the PSA system.
In U.S. Pat. No. 4,743,276 a method and adsorbent used for separating CO is disclosed. This patent describes the use of zeolites ion exchanged with various transition metal cations as CO selective adsorbents which operate in a PSA system with an elevated adsorption temperature ranging from 50.degree. to 250.degree. C. The patent notes that these types of adsorbents are susceptible to water vapor. This problem is addressed by cooling the inlet gas stream under pressure to effectively condense out the water in the feed gas. The dry gas is then sent to the PSA.
In U.S. Pat. No. 4,914,076 at column 5, lines 41-46, a recognition that pretreatment of an unspecified type may be necessary for a copper based CO adsorbent.
U.S. Pat. No. 4,917,711 discloses copper based CO adsorbents which may have a zeolitic support for the copper compound-CO adsorbent.
U.S. Pat. No. 4,470,829 discloses an adsorbent for CO which has a carbon intermediate support layer. At column 4, lines 46-63, the patent describes the adsorbents stability in the presence of water.
U.S. Pat. No. 4,019,879 discloses a copper on zeolite support that is tolerant of moisture-containing CO feed gases. The patent at column 1, lines 31-45 describes the COSORB process of absorption in a liquid using temperature swing absorption. The drying operation is also operated in a temperature swing manner using adsorption. See CHEMTECH, May 1975, pp 308-311.
U.S. Pat. No. 4,587,114 details a method for separating CO from mixed gases. The adsorbent used in this process is obtained by impregnation of a carbon support with various copper salts. The adsorbent produced is claimed to be stable with respect to the water content in the feed gas owing to the hydrophobic nature of the carbon support. Thus, no special dehydration process is necessary.
The prior art has failed to address an adsorbent arrangement and corresponding process that preferentially absorbs CO in the presence of H.sub.2, CH.sub.4, CO.sub.2 and N.sub.2, while protecting cuprous compound active sites on the adsorbent from degradation by feed gas-borne water and potentially ammonia in a pressure swing adsorption operation. This problem is overcome by the present invention as set forth below.